The narrator uses the term kernel to refer to how Marlow tells a story and what interests him in a story. The kernel is the center of the story but what interests Marlow is not only the center truth but the shell around it.

The misty halo is referring literally to the mist surrounding the African coast but also to how the truth of a thing is surrounded by such a mist making it impossible to fully make out.

They are all connected in storytelling by letting the reader know that Marlow is interested in the shell around the story not only the kernel of truth which a story must have and that the truth of what lies within is shrouded by mystery.